Interpretive Summary: Ammonia is emitted from the manure at beef cattle feeding operations. As manure is deposited directly on the open feedlot pen surface, the urea present in the urine is converted through enzymatic processes into ammonia which is volatilized to the atmosphere. The enzyme responsible for converting urea to ammonia gas is called urease. A laboratory research project was conducted to evaluate the effectiveness of NBPT, a commercially-available urease inhibitor, for reducing ammonia emissions from beef manure. Beef manure was placed into small emission chambers and a urea solution was added every few days to simulate the continual urine deposition that would occur in the feedyard. NBPT application rates varied from zero to 40 kg/ha, and with or without simulated rainfall. Clean air was passed over the manure surface and the amount of ammonia emitted from the surface was quantified. The amount of ammonia emitted from the manure surface was less when simulated rainfall was added. Ammonia emission rates for the NBPT treatments were 26 to 33% of the non-NBPT treatments, demonstrating that NBPT was effective at reducing emissions from the manure surfaces in both wet and dry conditions. Based on these laboratory results, the use of NBPT appears promising for reducing ammonia emissions at beef cattle feedyards. Additional research is warranted to study the effectiveness of NBPT under long-term conditions in an outdoor setting.

Technical Abstract:
Social, economic, and environmental factors have prompted the desire to reduce global atmospheric ammonia emissions. A research project was conducted to assess the efficacy of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) for reducing ammonia emissions from simulated open-lot beef cattle feedyard surfaces. A mixture of beef cattle feces and urine (manure) was placed into small emission chambers (16.7 × 16.7 × 17 cm deep). A urea solution was added every 2 d to simulate continual urine deposition in the feedyard. Clean air (1.4 L/min) was passed over the manure surface, and ammonia was trapped in an acid solution. The six treatments (3 reps per TRT) included combinations of NBPT application rate with or without simulated rainfall. NBPT was applied at zero, steady (5 kg/ha every 4 d), or increasing (5 kg/ha initially, doubled every 4 d up to 40 kg/ha). Rainfall treatments received 0.6 cm every 4 d. For all treatments, mean ammonia emissions from the manure were lower (P<0.05) when simulated rainfall was added. Mean ammonia emission rates for the NBPT treatments were 26 to 33% of the non-NBPT treatments, demonstrating that NBPT was effective at reducing emissions from the manure surfaces in both wet and dry conditions. There were no statistical differences in mean ammonia emission rates for the NBPT applied between the steady rate or increasing rate, showing that a steady NBPT application of 5 kg/ha every 4 d was effective in reducing ammonia emissions from the manure. The use of NBPT appears promising for reducing ammonia emissions at beef cattle feedyards. Additional research is warranted to study the effectiveness under long-term conditions in an outdoor feedyard setting.